Environ. Sci. Technol. 2008, 42, 2879–2885
The Rate of 2,2-Dichloropropane Transformation in Mineral Micropores: Implications of Sorptive Preservation for Fate and Transport of Organic Contaminants in the Subsurface HEFA CHENG AND MARTIN REINHARD* Department of Civil and Environmental Engineering, Stanford University, Stanford, California 94305-4020
Received November 16, 2007. Revised manuscript received January 22, 2008. Accepted January 24, 2008.
Nanometer scale pores are ubiquitous in porous geologic media (soils and sediments). Sorption of organic contaminants in micropores (e2 nm) can inhibit their hydrolytic transformation due to the limited availability of reactive water within hydrophobic micropore spaces. As a test case, we studied the dehydrohalogenation of 2,2-dichloropropane (2,2-DCP) sorbed in the micropores of several model mineral solids. In the micropores of a hydrophobic dealuminated Y zeolite, CBV-780, 2,2-DCP dehydrohalogenation proceeded significantly slower than in bulk aqueous solution and eventually stopped. This was attributed to the depletion of reactive water molecules in the micropore spaces. The 2,2-DCP sorbed in the micropores of more hydrophilic solids (aquifer sediment, aquifer sand, and silica gel) also transformed slower than in aqueous solution, and the reaction no longer followed first-order kinetics. Results of transport modeling support that reactive contaminants sorbed in microporous minerals can be preserved over geological time scales under conditions that limit desorption. This study shows that hydrophobic micropores in geological media may act as an important sink for anthropogenic organic contaminants in the subsurface, and that sorption in micropores may significantly increase the persistence of the sorbed contaminants.
Introduction The concept of sorptive preservation, i.e., protective sorption of otherwise labile organic matter (OM) to mineral surfaces, has been used to explain the persistence of labile OM in marine sediments (1–4) and soils (5–9). Such protection may involve physical shielding of the substrates from reactive agents or microorganisms in nano- to micro-scale pores formed by, for example, aggregation of clay platelets (10, 11). Sorption of biomolecules in the nanometer to submicrometer pores in zeolites, feldspar, and silica minerals, where they were protected from hydrolytic and photochemical destruction, might have played a role in the evolution of early life on Earth (12, 13). Recent experimental studies have shown that biopolymers sorbed in mesoporous (2–50 nm) solids were protected from biological degradation due to exclusion of bacteria and their extracellular enzymes (14, 15). Reduced * Corresponding author phone: (+1) 650 723-0308; fax: (+1) 650 723-7058; e-mail:
[email protected]. 10.1021/es702888h CCC: $40.75
Published on Web 03/12/2008
2008 American Chemical Society
bioavailability and biodegradation of organic contaminants and low-molecular-weight polymers was also noted after sorption onto mesoporous soils and synthetic solids (16, 17). Hydrolytic transformation of organic molecules, such as 2,2dichloropropane (2,2-DCP), was inhibited when sorbed in mineral micropores (e2 nm), presumably because contact with water molecules was limited in confined spaces of molecular dimensions (18, 19). Consistent with this hypothesis are the field observations that indicate persistence of hydrolyzable pesticides, such as 1,2-dibromoethane (20) and carbaryl (21), in low organic carbon surface soils. Similarly, organic phosphates, which hydrolyze quickly in aqueous solution, have been found to persist in buried soils for thousands of years (22). Chlorinated aliphatic hydrocarbons (CAHs) are frequently found groundwater contaminants (23) that hydrolyze in water with half-lives ranging from days to billions of years, depending on structure (24). For some CAHs, as for many other pollutants, hydrolysis is an important degradation process and knowing the factors that affect the rates is important for assessing their environmental risk. Micropores are abundant in porous geological media (soils and sediments), typically in the surface microstructures of minerals formed through weathering, precipitation, or turbostratic stacking of nanosized particles, as well as in the nanometer-scale structural pores, cavities, and channels of crystalline and amorphous minerals (18, 25). Combined, these nanometer-scale pores can amount to a significant sink for hydrophobic contaminants in the subsurface environment (19, 26–29). Diffusion of molecules in micropores is hindered and consequently uptake and release of organic contaminants from these micropores occur at distinctly longer time scales compared to surface adsorption or partitioning into organic matter (18, 19, 28, 30). The sorptive preservation of low molecular weight hydrolyzable compounds in mineral micropores has not yet been evaluated in detail. We have previously demonstrated that sorptive preservation of 2,2-DCP increases with micropore hydrophobicity (i.e., decreasing water affinity) (19). Here, we present data indicating that 2,2-DCP dehydrohalogenation in micropores not only proceeds significantly slower than in bulk aqueous solution but eventually comes to an apparent standstill. Through modeling contaminant behavior in a hypothetical aquifer we show that sorptive preservation in micropores can cause reactive contaminants to persist for geologic times. 2,2-DCP was selected as a model contaminant because in water it dehydrochlorinates to 2-chloropropene (2-CP) at a relatively fast rate independent of pH (24, 31). This reaction exhibits first-order kinetics with a rate constant of 4.88 × 10-6 s-1 at 24 °C and an activation energy of 111.1 ( 2.0 kJ/mol (24). The hydrolysis half-lives (t1/2) of 2,2-DCP at 24 and 50 °C are 39.5 and 1.06 h, respectively. 2,2-DCP adsorbs into water-filled micropores by displacing loosely bound water from the hydrophobic micropore spaces, forming microclusters of 2,2-DCP molecules (19, 32). In these clusters, 2,2-DCP molecules contact each other, the tightly bound water molecules, and the hydrophobic pore wall, but hardly any mobile (i.e., reactive) water (Figure S1a, Supporting Information). Factors that can be postulated to inhibit 2,2-DCP transformation are the absence of solvation by water (which prevents charge separation, the initial step of dehydrohalogenation), hindered molecular diffusion, accumulation of reaction products (H+ and Cl-), and depletion of reactive water (due to binding of water molecules into the hydration shells of proton and chloride). VOL. 42, NO. 8, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Properties of Solids Used in This Study synthetic solid
natural
CBV-780
silica gel A
LLNL sediment
Borden sand
particle size, µm bulk densitya, g/cm3 porosityb organic carbon, wt.% major components
1–2 µm 0.22 0.92 0 dealuminated faujasite
BET surface area, m2/g micropore volume, µL/g
780 451 (pore diameter: 0.74–1.2 nm) 32
330 0.36 0.82
